by David Allen

Allen's main idea is that your brain loses productivity when it is bogged down by a million things to do, so by removing all of these things from your head you greatly increase the ability to get things done. A good analogy, and one he uses in the book, is when your computer tries to do too many things at once and the computer's memory or RAM fills to capacity resulting in slowness or a computer crash; your brain is like the computer and can't handle too many tasks at once.